This invention relates to a magneto-optical recording disc adapted for magneto-optical recording in the magnetic field modulation system.
Optical discs are of great interest as high capacity information bearing media. Among the optical discs, magneto-optical discs of the magnetic field modulation system are expected to find use in data files and other advanced applications. A magneto-optical disc is recorded in the magnetic field modulation system by directing a laser beam from an optical head to the recording layer of the disc in a DC manner for raising the temperature of the irradiated spot, and applying a modulated magnetic field across the recording layer from a magnetic head opposed to the optical head at the same time. Therefore, the magnetic field modulation system allows for overwrite recording.
Most of conventional magneto-optical disc drives depend on a contact start and stop (CSS) system using a flying magnetic head. The magnetic head is in contact with the disc surface at the start and end of disc rotation. Thus the disc on the magnetic head facing surface is provided with a protective coating for preventing the magnetic head from adsorption and crushing.
For the protective coating on magneto-optical discs of the CSS system, various resin compositions were proposed. For example, Japanese Patent Application Kokai (JP-A) No. 301040/1990 discloses a resin composition predominantly comprising a fluorinated polyurethane resin and JP-A 37844/1991 discloses a resin composition predominantly comprising a fluoro-resin soluble in organic solvents. It is also proposed to incorporate metal elements and lubricants in the resin composition, the metal elements having greater ionization tendency than the transition metal element of which the recording film is made. JP-A 40149/1990 discloses a UV-curable resin having a lubricant blended therein, JP-A 17844/1991 discloses a lubricating layer, and JP-A 62338/1991 discloses an overcoat layer over which fine particulates of silicone resin or nickel are dispersed to uniformly distribute irregularities of 0.1 to 0.5 .mu.m high on the surface.
Magneto-optical discs capable of recording/reading operation at the same linear velocity as the compact disc (CD) are recently highlighted since they can share the drive unit with the CD simply by adding or modifying the optical system. The CD employs a linear velocity as low as 1.2 to 1.4 m/sec. at which the magnetic head cannot be float. Then the CSS system cannot be employed. One would attempt to use a stationary magnetic head which is held at a fixed distance from the disc surface. The stationary magnetic head, however, can collide with the disc due to axial runouts of the disc and vibration of the drive unit. Servo means must be provided in order to allow the head to follow axial runouts of the disc, resulting in a complex drive mechanism.
Under the circumstances, we reached the concept of a magnetic head in continuous contact with the disc surface and made research works on a magneto-optical disc capable of such operation. It was found that when the magnetic head was in sliding contact with the disc surface, the protective coatings previously proposed for the CSS system were unsatisfactory.
More particularly, the protective coating having a lubricant incorporated therein cannot retain lubricity for a sufficient time since the magnetic head is in continuous sliding contact with the protective coating surface. It is effective to increase the surface roughness of a protective coating by dispersing fine particulates therein. Although JP-A 62338/1991 discloses to disperse fine particulates of resin or magnetic material having a size of 0.2 to 1.2 .mu.m to provide irregularities of 0.1 to 0.5 .mu.m high, the roughness of this order on the protective coating is insufficient to reduce friction. It is then difficult to always maintain low friction between the magnetic head and the protective coating. Fine particulates of metals and ceramics have a sufficiently high hardness to cause damage to the magnetic head. It is to be noted that the average roughness referred to in JP-A 62338/1991 is probably in terms of Rz rather than Ra according to the inventors' knowledge though not specified in the disclosure.